Influence of surface anisotropy on the hysteresis of magnetic nanoparticles

We present the results of Monte Carlo simulations of the magnetic properties of individual spherical nanoparticles with the aim to explain the role played by surface anisotropy on their low temperature magnetization processes. Phase diagrams for the equilibrium configurations have been obtained, showing a change from quasi-uniform magnetization state to a state with hedgehog-like structures at the surface as $k_S$ increases. Through the simulated hysteresis loops and the analysis of spin configurations along them, we have identified a change in the magnetization reversal mechanism from quasi-uniform rotation at low $k_S$ values, to a non-uniform switching process at high $k_S$. Results for the dependence of the coercive field and remanence on $k_S$ and particle size are also reported.